1. Field of the Invention
The present invention relates to a thermosettable composition useful for preparing an anisotropic composite wherein the composition includes a thermosetting resin, a hardener, a filler having an aspect ratio higher than 5:1, and optionally, a catalyst; and a thermoset anisotropic composite product made from the thermosettable composition. The anisotropic composites made from the thermosettable compositions of the present invention may have improved mechanical and thermo-mechanical properties via orientation of the high aspect ratio fillers.
The thermosettable compositions of the present invention are useful in various applications such as casting, potting, and encapsulation, such as electrical and electronics applications, and composites.
2. Description of Background and Related Art
Epoxy systems containing fillers usually afford isotropic properties to the resulting thermoset composition and a degradation of some physical performance when such fillers are used in such thermosettable formulations. Anisotropy is an unusual phenomenon in some thermoset products such as epoxy composites. The property of being “anisotropic” in chemistry is a property of a substance that depends on direction; and is the property of being directionally dependent, as opposed to isotropy, which means homogeneity in all directions.
“Anisotropy” herein is defined as a difference in a physical property, including properties such as thermo-mechanical, coefficient of thermal expansion (CTE), mechanical properties and other properties, for a material when such material is measured along different axes.
It would be desirable in the industry to achieve anisotropic thermo-mechanical properties of thermoset composites filled with inorganic fillers, particularly when the thermoset matrix resin is an epoxy resin. Anisotropic properties of thermoset composites desired to be achieved include, for example, the thermo-mechanical properties, the CTE and the mechanical properties of the composite.
The prior art discloses several known composites having fillers incorporated into the composite. For example, WO 2002040577 teaches the coupling of reinforcing fibers to resins in curable composites.
WO 99-US1380 19990121 discloses filled compositions of syndiotactic monovinylidene aromatic polymer for molded articles having good mechanical properties, surface smoothness and heat aging stability.
JP 03247652 A discloses polyester compositions with good mechanical strength and heat resistance.
Cherevatskii, A. M.; Khozin, V. G.; Voskresenskii, V. A.; Shepelev, Yu. F. USSR. Plasticheskie Massy (1982), (5), 47-8, discloses wollastonite as an effective filler of epoxy polymers.
WO 2006-EP64291 20060714 discloses highly filled epoxy resin casting and potting compositions.
WO 2004065469 A1 20040805 discloses a highly loaded casting resin system containing fillers with multimodal particle size distribution.
WO 2003072646 A1 20030904 teaches highly loaded cast resin systems containing fillers of multimodal particle size distribution.
JP 2005082760 A 20050331 discloses poly(phenylene sulfide) resin compositions for precision moldings with less mechanical anisotropy and dimensional precision.
The compositions described in the above prior art relate to a highly filled system, normally having from 40%-60% filler (see, for example, WO 2003072646; WO 200406546; and WO 2007012577). The known compositions of a highly filled system are described as being useful as a composition for electrical casting. The filler shape of high aspect ratio and the anisotropic thermo-mechanical property and tensile strength are not described. A processing method to achieve anisotropic composite material properties is also not described in the above prior art.
U.S. Pat. No. 4,358,552 teaches epoxy resin molding compositions having low coefficient of thermal expansion and high, thermal conductivity. U.S. Pat. No. 4,358,552 describes a composite made of 40%-80% fillers, including the anisotropic filler like wollastonite. The CTE property is described, however, the improved tensile strength resulting from the orientated distribution of high aspect ratio filler is not described. The prior art document contains very high levels of filler, and the expected viscosity increase would prohibit the compounds use in casting applications. Also the prior art compositions are not expected to afford anisotropic behavior due to the very high filler loadings. Moreover, the processing method of the anisotropic composite is not described, and there is no disclosure of the composition being used in a casting application.
U.S. Pat. No. 6,001,902 discloses a wollastonite-containing curable epoxy resin. U.S. Pat. No. 6,001,902 describes a composition of epoxy resin and wollastonite as inorganic filler used for electrical casting and encapsulation. U.S. Pat. No. 6,001,902 discloses a loading of wollastonite above 50%, but does not describe the wollastonite shape and aspect ratio. The preferred orientation of wollastonite in the composite and the resulting anisotropic thermo-mechanical properties are also not described. The processing method of anisotropic composite is not described.
While the use of fillers in a polymeric system is known, composites having anisotropic properties obtained from casting using very high aspect ratio (>1:20) fillers is not reported in the prior art. More specifically, it is now known to use high aspect ratio fillers at relatively low concentrations to achieve properties, such as rigidity and CTE properties of polymeric matrices, comparable to low aspect ratio fillers at higher concentrations. Consequently, there is a need in the industry to develop a thermosettable composition that will provide a composite with a balance of properties such as viscosity/thermo-mechanical properties. In addition, there is a need in the industry to provide a thermosettable composition which can be used to prepare a composite material having anisotropic properties; and to provide a process for making such composite.